1. Field of the Invention
The present invention relates to vehicle speed sensor failure detection diagnostics.
2. Background Art
In the heavy duty vehicle industry there is widespread interest in limiting vehicle speed to improve fuel efficiency. In particular, trucking fleets can reduce costs and encourage safe vehicle operation by limiting the maximum allowable speed of a vehicle. Vehicle operators generally oppose and resist use of vehicle speed limiting systems because they tend to increase the time required to transport shipments that may marginally reduce their efficiency and productivity. Some operators have been known to attempt to frustrate the vehicle speed limiting devices and systems by damaging, disconnecting or modifying such devices and systems.
Governors were originally proposed for speed limiting that were integrated with vehicle fuel systems to limit the quantity of fuel supplied to a vehicle engine. These systems were easy to remove or re-calibrate rendering them ineffective.
With the introduction of electronically controlled engines more sophisticated approaches to vehicle speed limiting have been developed. Algorithms have been proposed for such control systems that provide effective vehicle speed limiting without limiting the engine speed so that the full engine power is available in low gear ranges. These sophisticated systems require multiple inputs that make it substantially more difficult to disable or override vehicle speed limiting systems.
One approach to vehicle speed limiting systems is proposed in U.S. Pat. No. 5,896,083 to Weisman that is assigned to the assignee of the present application. The Weisman speed limiting system monitors the vehicle speed, engine speed and transmission and gear selection to provide a vehicle speed limiting system that may be easily implemented in electronically controlled engines. One problem with this proposed solution is that the system is subject to false indication of system inoperability or tampering at low vehicle speeds.
Another approach to vehicle speed limiting is proposed in U.S. Pat. No. 6,573,827 to McKenzie that is also assigned to applicant's assignee. This patent proposes the use of a global positioning system (GPS) to monitor vehicle speeds. A first GPS signal indicating vehicle location is checked and then compared to a second GPS signal at a subsequent predetermined time period. If the time required to travel from the first GPS location to the second GPS location indicates that the vehicle is proceeding at a greater rate of speed then the maximum allowable speed, the engine control can be used to limit the engine speed to prevent the vehicle from traveling at a speed that is greater than the maximum allowable speed. This is a complicated system that requires the use of global positioning signals that communicate with the GPS of the vehicle. It may also be subject to false indications of tampering or the like if the GPS signal is subject to signal interference.
Inaccurate vehicle speed fault logging can also occur as a result of some vehicle service procedures. When a fault is logged by an engine control system the source of the fault must be determined that requires additional service operations. For example, in some instances a stall test may be performed in which the engine is checked to determine if the engine is producing rated power output. In the stall test the engine is run with the transmission in its converter mode to determine if a speed range target may be attained within a set time period. When a stall test is run on a vehicle a false vehicle speed fault code may be logged in the engine control system.
Another service operation that may result in false vehicle speed fault code logging is an engine air restriction test. Air restriction tests may be used to verify proper engine operation upon completion of a service procedure. After replacing an air filter, the service facility may conduct the restriction test by placing the transmission in neutral while the engine is operated through a specified engine speed level. According to the air restriction test procedure, the engine is run at the specified speed to activate an engine diagnostic parameter that may eliminate the need to conduct a road test to verify that the service operation has been successful. Previously, erroneous vehicle speed default codes were logged for the vehicle speed limiting system when conducting restriction tests.
False vehicle speed limiting system errors may also be generated when a vehicle is operated in the converter mode (when the torque converter is not locked up). This may occur when a vehicle is operating in off road conditions in which sand, snow, ice, or mud may result in the wheels slipping that in turn causes the transmission to be shifted into its converter mode. When in the converter mode the engine speed and fueling rate may be sufficient to trigger an erroneous fault log if the vehicle speed is below a predetermined level.
These and other problems are addressed by applicant's invention as summarized below.